pampering to the xenophobic tendencies of the Daily Mail, Telegraph, Express et al who can't bear the thought of people crossing the UK border in the same, uncomplicated way that they do virtually all other (western) european borders.
Key difference being there's millions more displaced young men who want to come here mate. Look at Sangatte, the reason the UK is a magnet for randoms to vanish into the black economy is that there's enough Africans,Pakistanis, Iraqis and Afghans here already that can support them quite against the law. Not quite the same in rural Belgium now is it? If you have an out of control immigration policy, you're not going to fix that with open borders!